The invention relates to a vehicle with an all-wheel drive and to a method for determining a road friction coefficient.
In vehicles with a hybrid drive system, comprised of an internal combustion engine and an electric machine, the electric machine is able to induce in a recuperation mode the kinetic energy of the vehicle into electric energy onto the vehicle wheels while at the same time imposing a recuperation or drag torque. In a vehicle with all-wheel drive system, therefore, not only the drive torque but also concept-dependent portions of the drag torque can be transferred onto the front axle and/or rear axle. These drag torques may cause high brake slip especially when a road surface has a low friction coefficient. This in turn results in a decrease of the cornering force and thereby in an unstable driving behavior.
DE 103 04 813 A1 discloses a generic type of a vehicle with all-wheel drive system. The drive system has an interaxle differential which distributes the driving torque to a front axle differential and a rear axle differential, via which the front wheels and the rear wheels are driven. The interaxle differential has a clutch which influences the driving torque distribution. The clutch can be operated in dependence on operating parameters of the vehicle for variable distribution of the drive torques.
The front-axle and rear-axle transmissions are translated differently in DE 103 04 813 A1. These differently translated axle drives cause a differential number between the axles, when the clutch is actuated with a differential torque. The differential torque can be implemented in deviation to the DE 103 04 813 A1 and also in the invention optionally also without the use of such a clutch or different axle ratios.
To carry out ESP control or ABS control, the provision of wheel sensors is already standard in modern vehicles to ascertain the rotational speeds of the front and rear wheels during normal driving. As however the actual road friction coefficient and the recuperation torque that can at most be transferred onto the road surface are not known, the recuperation capability of the hybrid drive and thus the possible efficiency advantage is limited in each driving situation so as to prevent an unstable driving behavior. As a result, storage in an on-board electrical system of the electric energy recovered in during the recuperation mode is inevitably limited.